The invention is related to precursors for lithium transition metal oxides for use in a rechargeable battery that have a unique combination of characteristics to provide excellent battery materials for demanding technologies such as automotive applications. More in particular, the precursors may be hydroxide or oxyhydroxide compounds having physical and chemical characteristics such as sulfur content and specific surface area that are related by a mathematical formula to the Ni content of the precursor.
Due to their high energy density, rechargeable lithium and lithium-ion batteries can be used in a variety of portable electronics applications, such as cellular phones, laptop computers, digital cameras and video cameras. Commercially available lithium-ion batteries typically consist of graphite-based anode and LiCoO2-based cathode materials. However, LiCoO2-based cathode materials are expensive and typically have a relatively low capacity of approximately 150 mAh/g.
Alternatives to LiCoO2-based cathode materials include LNMCO type cathode materials. LNMCO means lithium-nickel-manganese-cobalt-oxides. The composition is LiMO2 or Li1+x′M1-x′O2 where M=NixCoyMnzAm (which is more generally referred to as “NMC”, A being one or more dopants). LNMCO has a similar layered crystal structure as LiCoO2 (space group r-3m). The advantage of LNMCO cathodes is the much lower raw material price of the composition M versus pure Co. The addition of Ni gives an increase in discharge capacity, but is limited by a decreasing thermal stability with increasing Ni content. In order to compensate for this problem, Mn is added as a structural stabilizing element, but at the same time some capacity is lost. Typical cathode materials include compositions having a formula Li1+x(Ni0.51Mn0.29Co0.20)1−xO2 (for example x=0.00-0.03, referred to as NMC532), Li1+x(Ni0.38Mn0.29Co0.33)1−xO2 (for example x=0.08-0.10, referred to as NMC433), Li1+x(Ni0.6Mn0.2Co0.2)1−xO2 (x=0.02-0.04, referred to as NMC622) or Li1+x(Ni0.35Mn0.32Co0.33)1−xO2 (for example x=0.06-0.08, referred to as NMC111).
The target lithium-containing composite oxide is generally synthesized by mixing a nickel-cobalt-manganese composite (oxy-)hydroxide as a precursor material (having the same metal composition as the final cathode material will have) with a lithium compound and firing, and the cell characteristics can be improved by substituting a part of nickel, cobalt and manganese by other metal elements. As other metal elements Al, Mg, Zr, Ti, Sn and Fe are exemplified. The suitable substituting quantity is 0.1 to 10% of the total quantity of the nickel, cobalt and manganese atoms.
Generally, for the production of cathode materials with complex compositions, special precursors such as mixed transition metal hydroxides are used. The reason is that high performance Li-M-O2 needs well mixed transition metal cations. To achieve this without “oversintering” (high temperature sintering for a longer period together with a lithium precursor, typically Li2CO3 or LiOH) the cathode precursors need to contain the transition metal in a well-mixed form (at atomic level) as provided in mixed transition metal hydroxides, carbonates etc. Mixed hydroxides or carbonates are typically prepared by precipitation reactions. Precipitation of mixed hydroxides (for example, the precipitation of a flow of NaOH with a flow of M-SO4 under controlled pH) or mixed carbonates (for example, the precipitation of a flow of Na2CO3 with a flow of M-SO4) allows precursors of suitable morphology to be achieved. The precipitation typically takes place in a continuous stirred tank reactor (a CSTR reactor).
For characterizing a secondary lithium cell one of the most important parameters besides the discharge capacity is the irreversible capacity, which is responsible for the fading of the capacity during cycling. Lithium-excess layered transition metal oxides Li1+xM1−xO2 often have a huge irreversible capacity loss associated with the oxygen and lithium loss from the host structure of the layered oxide at the end of the first charging process. Although the irreversible capacity loss can be significantly reduced by coating with insulating materials (e.g., Al2O3 or MgO), the high surface area associated with the nanostructured lithium layered oxides could have such a high surface reactivity to induce side reactions between the electrodes and the electrolyte. This could lead to destabilization of the active materials and an increase in impeding passivation. Therefore, the electrolyte safety is of major concern, and ways have to be found to eliminate the side reactions and lower the irreversible capacity Qirr. As described by Lu and Dahn in “Layered Li[NixCo1−2xMnx]O2 Cathode Materials for Lithium-Ion Batteries”, Electrochemical and Solid-State Letters, 4 (12) A200-A203 (2001), for x=¼ and ⅜, when cycling between 2.5 and 4.4 V at a current of 40 mA/g, an irreversible capacity loss of 12% is quite acceptable. In the present invention however, the author achieve even better results. In patents such as U.S. Pat. No. 8,268,198 the relationship between the chemical composition of the precursor compound (i.e. the sulfate content) and the irreversible capacity of the lithium transition metal oxide cathode material has been established. A direct relationship between the physical characteristics of the precursor and the irreversible capacity of the lithium transition metal oxide cathode material, wherein also the Ni content of the material is taken into account, has not yet been provided.
It is expected that in the future the lithium battery market will be increasingly dominated by automotive applications. Automotive applications require very large batteries that are expensive, and must be produced at the lowest possible cost. A significant fraction of the cost comes from the cathodes, i.e. the positive electrodes. Providing these electrodes by a cheap process can help to lower cost and boost market acceptance. Automotive batteries also need to last for many years. During this time batteries do not always operate. A long battery life is related to two properties: (a) small loss of capacity during storage and (b) high cycle stability.
The automotive market includes different major applications. Batteries for EV (electric vehicles) need to store energy for several hundreds of km of driving range. Thus the cells are very large. Obviously the required discharge rates do not exceed a full discharge within hours. Thus sufficient power density is easily achieved and no special concern is paid to dramatically improve the power performance of the battery. Cathode materials in such batteries need to have a high capacity and a good calendar life.
Contrary to this, (P)HEV ((plug-in) hybrid electric vehicles) have much higher specific power requirements. Electrically assisted accelerations and regenerative braking require that the batteries are discharged or recharged within a couple of seconds. At such high rates the so-called Direct Current Resistance becomes important. DCR is measured by suitable pulse tests of the battery. The measurement of DCR is for example described in “Appendix G, H, I and J of the USABC Electric Vehicle Battery Test Procedures” which can be found at http://www.uscar.org. USABC stands for “US advanced battery consortium” and USCAR stands for “United States Council for Automotive Research”
If the DCR resistance is small, then the charge-discharge cycle is highly efficient; and only a small amount of ohmic heat evolves. To achieve these high power requirements the batteries contain cells with thin electrodes. This allows that (1) Li diffuses over only short distances and (2) current densities (per electrode area) are small, contributing to high power and low DCR resistance. Such high power batteries put severe requirements on the cathode materials: they must be able to sustain very high discharge or charge rates by contributing as little as possible to the overall battery DCR. In the past, it has been a problem to improve the DCR resistance of cathodes. Furthermore, it was a problem to limit the increase of DCR during the long term operation of the battery.
The present invention aims to provide improved precursors of lithium transition metal cathode materials for positive electrodes having an intermediate to high Ni content, made by a cheap process, having a reduced irreversible capacity Qirr upon cycling in the secondary battery, and having an improved DCR resistance.